Majorana fermions are predicted to localize at the edge of a topological superconductor, a state of matter that can form when a ferromagnetic system is placed in proximity to a conventional superconductor with strong spin-orbit interaction. With the goal of realizing a one-dimensional topological superconductor, we have fabricated ferromagnetic iron (Fe) atomic chains on the surface of superconducting lead (Pb). Using high-resolution spectroscopic imaging techniques, we show that the onset of superconductivity, which gaps the electronic density of states in the bulk of the Fe chains, is accompanied by the appearance of zero-energy end-states. This spatially resolved signature provides strong evidence, corroborated by other observations, for the formation of a topological phase and edge-bound Majorana fermions in our atomic chains.

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Observation of Majorana fermions in ferromagnetic atomic chains on a superconductor

A possible sighting of Majorana states Nearly 80 years ago, the Italian physicist Ettore Majorana proposed the existence of an unusual type of particle that is its own antiparticle, the so-called Majorana fermion. The search for a free Majorana fermion has so far been unsuccessful, but bound Majorana-like collective excitations may exist in certain exotic superconductors. Nadj-Perge et al. created such a topological superconductor by depositing iron atoms onto the surface of superconducting lead, forming atomic chains (see the Perspective by Lee). They then used a scanning tunneling microscope to observe enhanced conductance at the ends of these chains at zero energy, where theory predicts Majorana states should appear. Science, this issue p. 602; see also p. 547 Scanning tunneling microscopy is used to observe signatures of Majorana states at the ends of iron atom chains. [Also see Perspective by Lee] Majorana fermions are predicted to localize at the edge of a topological superconductor, a state of matter that can form when a ferromagnetic system is placed in proximity to a conventional superconductor with strong spin-orbit interaction. With the goal of realizing a one-dimensional topological superconductor, we have fabricated ferromagnetic iron (Fe) atomic chains on the surface of superconducting lead (Pb). Using high-resolution spectroscopic imaging techniques, we show that the onset of superconductivity, which gaps the electronic density of states in the bulk of the Fe chains, is accompanied by the appearance of zero-energy end-states. This spatially resolved signature provides strong evidence, corroborated by other observations, for the formation of a topological phase and edge-bound Majorana fermions in our atomic chains.

/pdf/observation-of-majorana-fermions-in-ferromagnetic-atomic-5451hode2h.pdf

Thin film nanoscale elements with a curling magnetic structure (vortex) are a promising candidate for future nonvolatile data storage devices. Their properties are strongly influenced by the spin structure in the vortex core. We have used spin-polarized scanning tunneling microscopy on nanoscale iron islands to probe for the first time the internal spin structure of magnetic vortex cores. Using tips coated with a layer of antiferromagnetic chromium, we obtained images of the curling in-plane magnetization around and of the out-of-plane magnetization inside the core region. The experimental data are compared with micromagnetic simulations. The results confirm theoretical predictions that the size and the shape of the vortex core as well as its magnetic field dependence are governed by only two material parameters, the exchange stiffness and the saturation magnetization that determines the stray field energy.

https://science.sciencemag.org/content/sci/298/5593/577.full.pdf

Direct Observation of Internal Spin Structure of Magnetic Vortex Cores

Spin mapping at the nanoscale and atomic scale

In electron-transfer processes, spin effects normally are seen either in magnetic materials or in systems containing heavy atoms that facilitate spin-orbit coupling. We report spin-selective transmission of electrons through self-assembled monolayers of double-stranded DNA on gold. By directly measuring the spin of the transmitted electrons with a Mott polarimeter, we found spin polarizations exceeding 60% at room temperature. The spin-polarized photoelectrons were observed even when the photoelectrons were generated with unpolarized light. The observed spin selectivity at room temperature was extremely high as compared with other known spin filters. The spin filtration efficiency depended on the length of the DNA in the monolayer and its organization.

https://science.sciencemag.org/content/sci/331/6019/894.full.pdf

Spin selectivity in electron transmission through self-assembled monolayers of double-stranded DNA.

Spin-polarized tunneling is demonstrated on Gd(0001) thin films using ferromagnetic probe tips in a low-temperature scanning tunneling microscope. The magnetic field dependent asymmetries in the differential tunneling conductivity are found at bias voltages which correspond to the energies of the spin components of the exchange-split Gd(0001) surface state. Maps of the spatial variation of the asymmetry reveal the magnetic structure of Gd(0001) thin films with a lateral resolution better than 20 nm. It is found that magnetic tip coatings thicker than 100 monolayers Fe may modify the sample domain structure due to the stray field of the tip.

Spin-Polarized Vacuum Tunneling into the Exchange-Split Surface State of Gd(0001)

Theoretical predictions of a new phenomenon arising in photoelectron emission from oriented molecules by circularly polarized light have been experimentally verified for CO. For a special geometry photoelectron-intensity differences occur upon reversal of photon helicity. The measured asymmetries (up to 80%) show good agreement with an ab initio calculation at photon energies between 20 and 40 eV. Theoretically, the new manifestation of circular dichroism is already obtained in the pure electric dipole approximation without inclusion of spin-orbit interaction.

Circular dichroism in the angular distribution of photoelectrons from oriented CO molecules.

Spin-polarization effects for electrons passing through thin iron and cobalt films

A magnetic dichroism in photoemission from ferromagnets induced by unpolarized light is observed for the first time in the valence band region as well as for surface states. The study was carried out with magnetized Fe(110), Co(0001), and Gd(0001) films grown epitaxially on W(110). This phenomenon leads to an intensity asymmetry for opposite directions of the sample magnetization. It can be easily used for a routine laboratory diagnostic of magnetic behavior. A simple theoretical interpretation of the phenomenon based on the atomic model is given.

Magnetic dichroism in photoemission with unpolarized light.

Circular dichroism in the angular distribution of photoelectrons (CDAD) has been observed in core-level photoemission from spherically symmetric initial states of nonmagnetic species utilizing circularly polarized soft-x-ray radiation from BESSY. Up to now, CDAD was predicted and observed only for aligned initial states. The data for oriented CO molecules prove that circular x-ray dichroism in photoemission from core levels is a general phenomenon that is not restricted to ferromagnets. High asymmetries of 50% suggest future applications as an effective circular x-ray analyzer.

Mathias Getzlaff

Papers

Spin-Polarized Vacuum Tunneling into the Exchange-Split Surface State of Gd(0001)

Circular dichroism in the angular distribution of photoelectrons from oriented CO molecules.

Spin-polarization effects for electrons passing through thin iron and cobalt films

Magnetic dichroism in photoemission with unpolarized light.

Circular dichroism in x-ray photoemission from core levels of nonmagnetic species